The invention pertains generally to actuation devices and is more particularly directed to multi-positional actuation devices which are controlled by an electronic digital processor.
For the regulation of the operation of many apparatus today there is necessitated the physical positioning of a control member according to a schedule or control law. Usually, the mechanical positioning of the control member is accomplished by an actuation device responding through an interface to a control signal calculated by a controller. Increasingly, the control signal will be calculated by a controller including a digital electronic processor which will regulate the positioning of the actuator as a function of a digital output word. Heretofore, the conversion of the information contained in the digital output word into a physical position has not been efficiently performed by conventional actuators.
Generally, either a digital-to-analog converter has been needed to control an analog actuator system positioning itself with respect to the analog signal or each bit of the digital control word is used to control an associated binary actuator determining a separate position. The former method is quite complex and requires relatively expensive actuators or stepper motors while the latter method restricts the number of physical actuations controlled to the number of bits contained in the output word. Neither actuator technique is entirely satisfactory for the sophisticated digital control systems being developed for future utilization.
More specifically the control of the operation of combustion engines based on their operating parameters may require many multipositional actuators. For example, to control a combustion engine of the gas turbine type an electronic controller has been used to control actuators for the bleed air valves of the compressor, the stator vane positioning, the nozzle positioning, fuel control, and positioning of other control members. Additional control functions may be envisioned for multipositional actuators in the future.
With respect to a particular application for a multiposition actuator, the bleed air valves are a series of valves used to vent the compressor of a gas turbine engine when the air flow between adjacent stages is mismatched to prevent the compressor from stalling. These valves are used primarily for starting or other operational sequences where one or more of the series is opened to regulate the quantity of air being supplied to the final compressor stages. The air bleed valves have also been used to provide a supply of pressurized air to other areas of the engine system.
Previously, each air bleed valve has been controlled by an individual solenoid. These solenoids were controlled by a digital word where one bit was assigned to each valve. The presence of a bit in the control word would actuate a particular air bleed and the absence of a bit would close it. However, this system mandates that each solenoid will be large or mechanically powerful enough to perform the task assigned. Since the solenoid valves work against compressor pressure they have been by necessity relatively heavy. The excess weight of these valves in gas turbine engine systems for aircraft is viewed critically.
Moreover, the use of these large solenoids requires that a separate driver circuit be used for each one. The logic level developed digitally have to be transformed into power levels sufficient to drive the large solenoids. These drivers consume considerable amounts of power which must be generated onboard in aircraft. The increased weight for aircraft of the electric generation devices used to create this excell power is also undesirable.